As a reactor for a vehicle-mounted converter, JP 2013-135191A discloses a reactor including a coil including a pair of wound portions (coil elements) formed by windings wound in a spiral shape, a magnetic core including a plurality of core pieces assembled in an annular shape, a bobbin interposed between the wound portions and the magnetic core, and a resin mold portion that covers a combined body of the coil and the magnetic core. The magnetic core includes a plurality of inner core pieces disposed inside each of the wound portions, a gap plate interposed between adjacent core pieces, and two outer core pieces disposed outside the wound portions. The above-described bobbin includes a pair of frame-shaped bobbins that are interposed between the wound portions and the outer core pieces, and that are provided with two through holes into which ends of the inner core pieces are inserted, and an inner side bobbin disposed in a coil interior space located between the wound portions and a columnar combination in which the plurality of inner core pieces and the gap plate are assembled.